Branching is usually performed by connecting different switches in series as described in the Siemens manual "Schalten, Schutzen, Verteilen in Niederspannungsnetzen" (Switching, Protecting, and Distributing in Low-Voltage Networks 1992 Edition, Table 3.17, p. 143), where a circuit breaker, a contactor, and an overload protection is connected between a motor and the voltage source supplying it with power. Three-phase power controllers, semiconductor relays, rectifiers, mechanical limiters, motor protection switches, and fuses, as well as indirect overload relays, are known as additional switching elements and components for controlling loads. Different switching sequences are performed with such low-voltage branching. These include operational switching of loads, phase-controlled or generalized phase controlled switching of motors, capacitors, and non-linear loads such as lamps and inductors. In the case of inductors, switching occurs instantaneously, e.g., at zero crossing or between zero crossings. Short-circuit currents in the branch circuit are intensity-limited to avoid harmful overloads. Another function of the low-voltage branch circuit is to detect short-circuit currents and shut them off in the shortest possible time taking into account the allowable overvoltage. It would also be desirable to avoid switching loads to short-circuits or to achieve the quickest possible shut-off if such switching occurs. Furthermore, instances of overload are detected and, if needed, shut off in the low-voltage branch circuits. The operating state of the branch circuit is reported for status monitoring.
European Patent No. 0 304 951 describes an electronic branch circuit switch for switching a load connected to a power supply source on and off, with two bidirectionally connected semiconductor switching elements connected in series with the load. No interrupting switching devices with an isolating distance are present in the load circuit.
U.S. Pat. No. 5,006,949 describes a device having a resistor and transistors, with which the current is switched off in the load circuit in the event of an overload. Furthermore, a control circuit designed as an optocoupler is described, which is capable of transmitting control signals to a semiconductor switching element without an electrical connection.
European Patent No. 0 421 891 describes a static isolating switch with electrical isolation, where a load having an upstream semiconductor switching element is supplied with direct current. The semiconductor switching element shuts off the current under normal conditions, as well as in the case of an overload or short-circuit. In the event of failure of the semiconductor switching element, the isolating switch in the load circuit is opened. An electromagnet connected in parallel to the load is used for triggering; the magnet armature position provides the opening command to the isolating switch. When the load is supplied from an AC source, two bidirectionally connected transistors are used instead of a semiconductor switching element.
German Patent No. 42 09 167 describes a switching and monitoring unit for electric drives. The unit is used, in particular, for switching and monitoring electric drives with operating voltages over 60 V, in particular for auxiliary and secondary drives, with the electric drives featuring switching elements, e.g., circuit breaker elements, protective switching elements, and current rectifiers. Instead of mechanical switches such as isolating switches and motor protection switches, semiconductor switching elements are connected upstream for switching in this case, and the actual operating status is picked up and conveyed to monitoring elements or circuits.